Muscle stem cells are responsible for the postnatal muscle mass gain and muscle regeneration. Muscle stem cells are non-tumorigenic in vivo. The application of muscle stem cells have no ethical problems faced by embryonic stem cells. They therefore display wide application prospects to treat various muscle degenerative diseases such as muscle wasting and dystrophy etc. One of the major obstacles preventing the application of muscle stem cells in clinic is the lack of method to support long term muscle stem cell culturing in vitro. Similar to the majority of adult stem cells, muscle stem cells can only divide for 2-4 times in the in vitro culturing system and can hardly be serially expanded.
The current protocols to culture muscle stem cells (satellite cells) are largely all based on the method established by Dr. Bischoff in 1986. The method was further optimized by Dr. Beauchamp et al. and it cultures muscle stem cells in F10 basal culture medium supplemented with 10 ng/ml FGF. Under this culture condition, muscle stem cells can divide 3-5 times. When muscle stem cells are continuously expanded, they will go through a major crisis. During the major crisis, most of the cells die. The survived cells differentiate to progenitor cells losing the stemness. Therefore, after culturing in vitro for 1-3 passages (3-12 days), all the isolated muscle stem cells differentiate to progenitor cells losing the stemness. And the molecular markers of muscle stem cell cannot be detected in these progenitor cells. These progenitor cells show very low efficiency to repair muscle injury in vivo. They are unable to form functional stem cells in vivo after injury reparation and fail to repair secondary injury after transplantation. (Bischoff P. A satellite cell mitogen from crushed adult muscle. Dev Biol., 1986. 115(1): 140-147. Beauchamp, J R, et al., Expression of CD34 and Myf5 defines the majority of quiescent adult skeletal muscle satellite cells. J Cell Biol. 2000. 1546): p. 1221-34.) That is, the muscle stem cells will go through a crisis phase if the current in vitro culturing system of muscle stem cells is applied. After the crisis, the majority of the cells with the ability to proliferate have become muscle progenitor cells and lose the ability to repair muscle injury in vivo. Due to the presence of the said problem, the genetic correction in muscle stem cells to treat muscle degenerative diseases is difficult to be applied in clinic.